Testing environments for mobile phones and other communication devices may require large numbers of expensive apparatus. Among many other testing apparatus, radio frequency (RF) carrier elevation and RF channel simulation are particularly complex and expensive due to their bandwidth, high frequency, and data rate requirements. Among these apparatus, the RF channel simulator is usually the most expensive since it has a very large bandwidth and strong noise constraints.
Additional requirements and constraints are related to the details of the communication system under test. For example, a frequency hopping spread spectrum communication system changes its signal transmission frequency within its transmission frequency band at regular time intervals. The changes in signal transmission frequency are done according to a pseudo random pattern called a “hopping” pattern. For example, a BLUETOOTH™ wireless network uses 79 frequencies, and in basic operation hops between frequencies every 625 microseconds, which is equivalent to 1600 hops per second.
FIG. 1 represents the relation 100 between frequency hopping, the frequency band, and the channel. Channel profile 102 represents the level of attenuation at a given frequency within the channel frequency band. In other words, channel profile 102 indicates the frequency response of the channel. The channel profile 102 simulates the properties of the transmission medium that may attenuate signal transmission. For wireless transmission, the channel profile 102 is a simulation model of air, rain, water vapor, buildings, interference, and other channel properties. A transmission medium may have fixed or varying properties, so a channel profile may have a fixed or varying attenuation over time. Fixed or varying attenuation over time is also known as “fading”, and a simulation model with fading is known as a “fading model”. Examples of fading models are the Ricean, Rayleigh, or Gaussian fading models, which can be found, for example, in Typical Urban (TU), Hilly Terrain (HT), Rural Area (RA), Equalizer Test (EQ) in the Global System for Mobile Communications (GSM) specifications: 3GPP-TS-05.05—3rd Generation Partnership Project, Technical Specification Group GSM/EDGE, Radio Access Network, Radio Transmission and Reception.
Signal band 104 represents the band of a signal centered at a frequency 106 offset by “f0” 107 from the carrier frequency 108 before signal frequency hopping. Signal band 110 represents the band of the signal centered at a new frequency 112 offset by “f1” 109 from the carrier frequency 108 after signal frequency hopping. After the signal frequency hopping takes place, the attenuation of the signal band 110 changes due to the new frequency location of signal band 110 relative to the channel profile 102. The signal is heavily attenuated at frequency offset “f0” 107 relative to the attenuation at frequency offset “f1” 109. By using signal processing such as interleaving and error control coding, signals transmitted at frequencies 106 and 112 can be combined to produce a consistent data rate despite the different attenuation.
Because a wireless channel is usually in constant flux due to interference and motion, it is difficult to predict the location or locations of deep attenuation in the channel band. By using a signal that hops over many frequency offsets throughout the channel band along with signal processing, a consistent data transmission is possible. This is known as frequency diversity. For accuracy, a channel simulation must calculate the frequency attenuation of the channel profile at each hop frequency and at each simulated time interval, which is very computation intensive. Consequently, such simulations typically require equipment with high computational capabilities.
An example of a typical simulation and testing apparatus 200 including an RF channel simulator is depicted in FIG. 2 where baseband processing module 202 performs conventional baseband signal processing such as channel formatting, segmentation, interleaving, error correction coding, D/A conversion, etc. RF carrier elevation and hopping module 204 performs the frequency up conversion and signal frequency hopping. The RF channel simulator module 206 performs the simulation of the channel model. The downlink 208 represents the simulated signal converted into an actual RF radio signal (the details of the conversion are omitted for clarity), and fed to the test device 210. The test device 210 responds by generating and transmitting an uplink signal 212 back to the test apparatus. A conversion-to-baseband module 214 in the test apparatus converts the uplink actual RF radio signal into a digital signal, and then to baseband frequency for analysis.
A known alternative 300 to the typical simulation and testing apparatus depicted in FIG. 2 is to perform channel simulation together with the baseband processing module 302 as shown in FIG. 3, where the baseband processing module 302 performs conventional baseband signal processing such as channel formatting, segmentation, interleaving, error correction coding, D/A conversion, etc. In addition, the baseband processing module 302 also performs signal frequency hopping and full frequency band channel simulation. The RF carrier elevation module 304 performs frequency up conversion. The downlink 306 represents the simulated signal converted into an actual RF radio signal (the details of the conversion are omitted for clarity), and fed to the test device 308. The test device 308 responds by generating and transmitting an uplink signal 310 back to the test apparatus. Conversion to baseband module 312 in the test apparatus converts the uplink actual RF radio signal into a digital signal, and then to baseband frequency for analysis.
A limiting factor for the alternative depicted in FIG. 3 comes from the signal frequency hopping that must be performed before the full frequency band channel simulation in the baseband processing module 302. The bandwidth required at the baseband side for the channel simulation is determined by the full frequency bandwidth on which the generated signals hop, which is higher than the bandwidth of the signals generated, requiring a non-negligible increase in data rate and complexity, which results in a cost increase. Although usually less expensive than an RF channel simulator, this alternative increases the complexity of the baseband processing 302 and requires a more complex baseband and consequently an additional cost increase.
Another known test apparatus uses RF frequency hopping and baseband channel simulation but approximates the simulation of channel fading. The approximation uses an independent fading channel simulation for each hopping frequency in the hopping frequency set. The consequence is that all fading channels are computed and updated in parallel to account for time variation of the fading channels which adds complexity and, therefore, is computationally expensive. Because of the complexity, this solution is limited to a small number of hopping frequencies (e.g., only four), so it is not adapted to a more realistic scenario such as testing 10 target frequencies and time varying channel fading. Furthermore, the channel fading is not accurately simulated because the separately modeled fading channels do not accurately model the statistical properties of the full channel model such as the coherence bandwidth.